1. Field of the Invention
The present invention relates to an automatic document feeder for use in an image forming apparatus such as an electronic copying machine.
2. Description of the Related Art
Various automatic document feeders for use in electronic copying machines have been put to practical use. An automatic document feeder of this type is designed to feed document sheets placed on a tray, one by one, onto the document table (i.e., platen glass) of an image forming apparatus, and also to feed each document sheet from the table after the image formed on the sheet has been copied on a copy sheet. The feeder has a register roller, a document detector, and a feed roller. The register roller is rotated at a high constant speed to feed document sheets one after another toward the table at high speed. The document detector outputs a pulse upon detecting the rear end of any sheet being fed toward the document table. In response to each pulse output by the detector, the feed roller is rotated a predetermined angle and feeds the document sheet for a predetermined distance, thereby placing the sheet at a prescribed position on the document table. The automatic document feeder, however, is disadvantageous due to the following four drawbacks.
First, because the feeder has no means for moderating the inertia of each sheet being fed at high speed by the register roller, document sheets are often fed at different speeds and pass by the detector at irregular intervals. In this case, each sheet cannot be placed at the predetermined position on the document table, and the image on the sheet will inevitably be copied at a wrong position on the copy sheet.
Second, the document cover for holding a document sheet on the document table is expensive because it has a wide endless belt for feeding documents sheets and also a drive mechanism for driving the endless belt. Its cost is more than half the cost of the automatic document feeder as a whole.
Third, since the wide endless belt is put in contact with virtually the entire surface of the document table, the friction between the belt and the table is so great that the drive mechanism consumes much power to drive the endless belt, and the endless belt makes much noise while being driven in contact with the document table.
Fourth, since the document cover contains electric and electronic components, a harness must be used to connect the cover to the image forming apparatus. Due to the use of the harness, it is difficult for remove the cover for inspection and maintenance purposes.